Known roller-mills in which the rolls have smooth external surfaces do not mix and plasticise the material in the roll nip satisfactorily because such mixing and plasticising is not effected uniformly. If relatively high adhesion forces are produced between relatively hot portions of the material being processed and the roll surface, these hot portions of material are sharply drawn into the roll nip and are hard-sheared. In such a case, very high, but localised, temperatures are produced in the material whilst, in axially adjacent regions, colder portions of the material remain in front of the roll nip for an uncontrollably long time. This is because there is too little external frictional adhesion of the material with the roll surfaces to permit them to enter the roll nip.
These particles of materials are often referred to as "cold tongues" or "cold projections" and have often only cooled in their surface regions. Internally, however, the particles may well have been thermally decomposed because of their long dwell time in a heated region before they are drawn into the roll nip.